Cholesterol as a Modulator of the HIV-1 gp41 Fusion Domain's Function

Abstract

The early steps in HIV-1 infection of cells involve fusion of the envelope with the host cell membrane. The N-terminal fusion domain of the viral envelope glycoprotein 41 subunit is widely accepted to play a key role in the fusion process facilitating membrane anchoring, destabilization, and bending. Ability of the fusion domain to perform these essential functions depends strongly on the structural and mechanical properties of the host cell membrane, which are defined by the lipid composition and cholesterol content. Here we present results of an X-ray study aimed at the understanding the effect of cholesterol concentration in model lipid membranes on the activity of the viral fusion domain. Lipid monolayers at the air-liquid interface composed of DPPC and cholesterol were used to model an approximate environment where the fusion domain comes into contact with the host cell membrane. The electron density profiles across the films, derived from X-ray reflectivity data, demonstrate that the fusion domain penetrates into all DPPC/cholesterol monolayers. The depth of membrane insertion and orientation/conformation of the fusion domain within the film, as well as lipid-to-fusion peptide ratio, depend strongly on the membrane cholesterol concentration. Distinct insertion modes also suggest that the fusion domain-induced membrane curvature is considerably different in bilayers with low and high cholesterol content. Finally, using grazing incidence X-ray diffraction we have demonstrated that the viral fusion domain possesses limited membrane destabilizing effect and is incapable to degrade the gel phase in the DPPC/cholesterol films.